In four and six cylinder semi-hermetic reciprocating compressors used in refrigeration and air conditioning applications it is common to locate the cylinders in pairs or banks with the two cylinders in each bank 180.degree. out of phase with each other. The cylinders of each bank would have a common suction and discharge manifold structure. As a result, a single actuator is used to unload an entire bank of cylinders at the same time. However, because the cylinders are paired, there is a large increment of unloading. Specifically, the unloading steps are 50% for a four cylinder compressor and 33% for a six cylinder compressor. It is desirable to be able to have smaller steps of unloading by unloading a single cylinder at a time. There are certain difficulties associated with the unloading of one cylinder of a bank.
Suction cutoff is the preferred method of unloading and it effectively transforms a cylinder from acting as a compressor to acting like a vacuum pump. In acting as a vacuum pump, there is a comparatively significant amount of input work done on what was the suction stroke since the cylinder is drawn into a vacuum. The vacuum then provides an assist for a large portion of the revolution on what was the compression stroke. As a result, both cylinders of the bank have an overlapping work stroke rather than being 180.degree. out of phase since the discharge stroke of the compressor cylinder is paired with the suction stroke of the vacuum pump cylinder. When unloading one cylinder of a bank by suction cutoff, the result is a significant surge in the RPM at the completion of the work stroke which produces unacceptable machine vibrations.
Gas bypass is an alternative method of unloading a compressor and involves the direct return from the discharge side of the cylinder head to suction. Because some work is being done in the compression of the gas and because the cylinder is not acting as a vacuum pump, the bank remains more closely balanced although some of the work is otherwise unnecessary. Additionally, there is a heating of the gas since it is being partially compressed and then directly returned to suction rather than passing through the refrigeration equipment.